Part of "How It Gets Built" — Buildermuse's series on the machinery behind modern construction.
Ascent in Milwaukee stands 284 feet tall — 25 stories of apartments held up by wood — and the crew topped it out with a peak erection pace that concrete framers in that market couldn't touch. I've spent 30 years watching concrete decks cure at one floor every five to seven days, so when a superintendent tells me his mass timber crew set a floor in under a week with two-thirds the bodies, I go look for myself. Here's the deal: mass timber construction isn't a boutique experiment anymore. It's a system with its own logistics, its own trades, its own cost math, and about 2,500 mass timber projects built or in design across the U.S. according to WoodWorks' project tracker. If you bid commercial work, you're going to see one of these jobs land in your market within the next 24 months.
What CLT and Glulam Actually Are
Strip away the marketing and mass timber is two products doing two jobs. Cross-laminated timber — CLT — is the flat stuff: layers of dimension lumber, usually 3, 5, 7, or 9 plies, glued up with each layer running 90 degrees to the one below it. That cross-hatching is the whole trick. Solid sawn lumber is strong along the grain and weak across it; CLT is strong both ways, which turns a stack of 2x6s into a structural floor panel. Glulam — glue-laminated timber — is the linear stuff: beams and columns built from laminations all running the same direction, which is how you get a column that carries 25 stories of load.
Panel sizes and why they matter to your schedule
A typical CLT floor panel runs up to 10 feet wide and 40 to 60 feet long, with some presses producing panels 12 feet by 60 feet. Thickness for a residential tower floor lands around 6 to 8 inches for 5-ply and 7-ply layups. One panel covers 400 to 600 square feet of deck in a single crane pick. Compare that to forming, rebar, pour, and cure on a concrete deck and you start to see where the schedule money hides. On Ascent, the frame used roughly 2,400 cubic meters of timber, and the panels arrived pre-cut on CNC machines with the MEP penetrations already routed — a 4-inch sprinkler penetration cut at the plant costs pennies; the same hole core-drilled in the field costs $75 to $150 and a schedule interruption.
Where the panels come from
The North American supply base is real now, and that changes bidding. SmartLam runs plants in Montana and Alabama with capacity in the range of 100,000 cubic meters a year combined. Sterling Structural in Illinois turned an old newsprint mill into CLT capacity serving the Midwest. Mercer Mass Timber picked up the former Katerra plant in Spokane — one of the largest CLT facilities in North America at roughly 140,000 cubic meters of annual capacity — plus operations in Arkansas. Think Wood counts more than a dozen North American mass timber manufacturers today versus three in 2015. Practical translation: five years ago your panels came from Austria with a 14-week lead time and ocean-freight risk. Today a Midwest job can source domestic with 8-to-12-week leads. You still order panels the way you order elevators — early, with shop drawings locked — because a late panel package stops the whole frame.
The Code and the Fire Question
Nobody builds a 25-story wood building on a variance anymore. The 2021 International Building Code created three new construction types — IV-A, IV-B, and IV-C — after the ICC's Tall Wood Building committee reviewed years of fire testing. The height limits are the headline: Type IV-A allows 18 stories and 270 feet, IV-B allows 12 stories, IV-C allows 9. Ascent, at 25 stories, got there under Wisconsin's process with performance-based design before the 2021 provisions were widely adopted, but the point stands — the code path now exists in black and white, and more than 30 states have adopted or are adopting the tall wood provisions.
The char layer, explained like a fieldman
Here's what surprised me when I first sat through the fire testing data. Heavy timber doesn't behave like stick framing in a fire. A 2x4 is kindling; an 8-inch glulam column is a log, and logs are hard to burn through. When mass timber burns, the outer surface chars at a predictable rate — roughly 1.5 inches per hour — and that char layer insulates the wood behind it, which keeps carrying load. Engineers size members with sacrificial thickness: if you need a 2-hour rating, you add roughly 3 inches of sacrificial wood beyond what the structure needs. USDA Forest Products Laboratory and partner labs ran full-scale compartment burns — including a 2017-2018 series in a two-story CLT structure — where fires burned out or were controlled while the structure held. Type IV-A goes further and requires noncombustible protection (usually 2 to 3 layers of Type X gypsum) over most of the timber, which is why an 18-story IV-A job can carry 3-hour ratings on its primary frame. I'm not telling you wood is magic. I'm telling you the 2-hour and 3-hour assemblies passed the same ASTM E119 furnace tests concrete assemblies pass, and the code officials who signed off on 18 stories saw the data.
The Erection Speed Math
This is the section that pays for reading the article. Mass timber's cost case is not the material — it's the calendar.
Floors per week versus concrete
A cast-in-place concrete tower in most markets cycles a floor every 5 to 7 days with a big crew: formwork carpenters, rodbusters, finishers, pump crew — call it 40 to 60 workers on frame days. A mass timber crew on a comparable floor plate sets a floor in 4 to 6 days with 10 to 14 people: a crane operator, riggers, and two or three panel-setting crews running screw guns and hand tools. WoodWorks' case studies routinely report erection crews around 25 to 30% smaller than concrete equivalents, and several projects — Brock Commons in Vancouver is the famous one — hit two floors per week once the crews found their rhythm. Brock Commons framed 17 stories of timber in about 10 weeks, roughly four months faster than the concrete alternative the university priced.
What a saved month is actually worth
Run the numbers on a $60 million, 200-unit tower. General conditions on a job like that run $250,000 to $400,000 a month. Construction loan interest at today's rates on a drawn balance costs another $200,000-plus a month late in the job. Rent on 200 units at $2,200 average is $440,000 a month of revenue that starts sooner. Save two months on the frame — a conservative read of the case-study data — and you've put $1.5 to $2 million back in the deal. And because the panels go in dry, there's no cure time before follow-on trades start; MEP rough-in chases the frame two or three floors behind instead of waiting on shoring removal.
The Cost Reality, Line by Line
Straight structure-versus-structure, mass timber usually prices at a 2 to 8% premium over concrete or steel — WoodWorks' cost comparisons and most developer pro formas I've seen land in that band. But the frame is maybe 15 to 20% of total project cost, and the premium erodes in three places.
Foundations get 20%+ lighter
Timber weighs about a fifth of what concrete weighs per structural volume. A mass timber superstructure typically comes in 25 to 40% lighter than the concrete equivalent, which cuts foundation loads enough to shrink footings, reduce pile counts, or — on bad soils — turn a deep-foundation job into a shallow one. On soft-soil sites I've seen the foundation savings alone claw back 1 to 2% of total project cost. Ascent's team reported meaningfully smaller foundations than the concrete scheme they originally priced.
Finish and schedule offsets
Exposed timber ceilings can delete the drop-ceiling and skim-coat scope in units — $6 to $10 per square foot in some markets — because tenants pay a premium to look at wood. Then stack the schedule savings from the last section. Run all three offsets and a 5% frame premium can wash to even or better on the right building type: mid-rise and tall residential, office with 30-foot grids, and anything where the developer markets the structure itself.
Where it still loses
Long-span retail, heavy industrial floor loads, and markets with dirt-cheap concrete labor. If you're in a town where flatwork crews are plentiful and CLT ships 900 miles, concrete still wins the bid. Check what's actually moving in your region on our live federal bids board — timber-eligible projects show up in GSA and military family-housing solicitations more every quarter.
What Changes On Site
Moisture is the new enemy
Concrete likes water; timber does not. Panels arrive at 12 to 15% moisture content and need to stay under roughly 19% to avoid problems, so mass timber jobs run a written moisture management plan: end-grain sealed at the plant, panel stacks tarped, standing water squeegeed off decks within hours, and moisture meters logged weekly. Some GCs budget $50,000 to $150,000 for temporary protection on a tower and consider it cheap insurance. The jobs that skip this step sand and refinish thousands of square feet of stained ceiling at $8 to $15 per square foot, in front of the owner, at the end of the job.
The trades that adapt — and win
Carpenters become panel-setters, and the good ones become foremen fast because the skill set is layout, rigging, and reading CNC shop drawings rather than cutting sticks. A four-person setting crew with a tower crane can place 25 to 35 panels a shift once they're dialed. MEP is where the discipline shows: penetrations get modeled and cut at the plant, which means coordination happens 4 to 6 months earlier than plumbers and tin-knockers are used to. Shops that can do prefab and BIM coordination eat on these jobs; shops that field-route everything bleed. If your subs already do data center work, they've got the prefab muscle — the coordination culture is the same one driving the projects in our piece on state data center tax incentives.
Insurance and lenders, the honest version
Builder's risk on mass timber still prices 20 to 50% above concrete in many markets, and some carriers cap their line or decline outright — the underwriting data pool is young. Water damage, not fire, drives most of the claims conversation. Lenders have mostly come around on completed-building valuation, but expect a couple of extra weeks of diligence and a request for the fire engineering package. Developers who've done one deal report the second one finances noticeably easier; the first project in any market carries the education cost, which is worth 25 to 75 basis points of friction. Budget the time, bring WoodWorks' technical staff to the underwriter meeting (they do this for free), and it closes.
Frequently Asked Questions
How tall can a mass timber building legally go in the U.S.?
Under the 2021 IBC, Type IV-A construction allows 18 stories and 270 feet. Type IV-B allows 12 stories and IV-C allows 9. Taller projects — like 25-story Ascent — use performance-based design with the local jurisdiction. More than 30 states have adopted or are adopting the tall wood provisions.
Is mass timber actually safe in a fire?
Tested assemblies carry 2-hour and 3-hour ratings under the same ASTM E119 standard used for concrete and steel. Timber chars at a predictable rate near 1.5 inches per hour, and the char layer protects the load-bearing wood behind it. USDA Forest Products Laboratory full-scale burn tests supported the ICC committee's decision to allow 18 stories.
How much more does mass timber cost than concrete?
Structure-to-structure, expect a 2 to 8% premium. Offsets — foundations 20%+ lighter, deleted ceiling finishes, and 1 to 4 months of schedule savings worth $400,000-plus per month on a mid-size tower — frequently bring the total project delta to zero or better on residential and office buildings.
How fast does mass timber go up compared to concrete?
Case studies report floors set in 4 to 6 days with crews roughly 30% smaller than concrete framing crews, versus 5 to 7 days per floor for cast-in-place. Brock Commons framed 17 timber stories in about 10 weeks. Follow-on trades start immediately because there's no cure time.
Who makes CLT in North America?
More than a dozen manufacturers, including SmartLam (Montana, Alabama), Sterling Structural (Illinois), and Mercer Mass Timber (Spokane, Arkansas — roughly 140,000 cubic meters of annual capacity at the Spokane plant alone). Domestic lead times now run 8 to 12 weeks versus 14-plus from Europe.
What's the biggest field risk on a mass timber job?
Moisture, not fire. Panels need to stay under about 19% moisture content, so plan on $50,000 to $150,000 of temporary protection on a tower job and a written moisture management plan with weekly meter readings.
Your Action Item for This Week
Pull the last mid-rise you bid — anything 6 to 12 stories — and price the frame two ways. Call one CLT supplier (SmartLam, Sterling, and Mercer all quote budgetary numbers off a structural grid and a floor count, usually within 10 business days, free) and ask for dollars per square foot of deck delivered to your zip code. Then put your real general conditions number against a schedule that's 6 to 8 weeks shorter. If the delta is inside 5%, you've got a capability worth pitching to every developer in your contact list — because in most markets, fewer than 3 general contractors have set a single panel, and the first mover gets the next decade of negotiated work. While you're at it, scan the commercial construction desk for where tall timber projects are landing next.
More from the How It Gets Built desk
Edited by Ray Kowalski, Methods & Machines desk. New teardowns weekly.
